Evidence of human-caused climate change over the past 50 years has been well documented. Global surface temperature has increased approximately 0.70 C over the past 50 years and much of that increase can be attributed to anthropogenic sources. Climate change is anticipated to affect human health largely by changing the distribution of known risk factors such as extreme heat episodes, floods, droughts, air pollution and aero-allergens, and vector- and rodent-borne diseases. In particular, an expected increase in the frequency, intensity, and severity of extreme heat episodes, will likely have a profound impact on the public's health. Changes in the levels of air pollutants such as particulate matter and ozone can potentially exacerbate the already severe effects of heat. Designing interventions and mitigation strategies to protect the public's health will require first developing a clear understanding of how extreme heat episodes affect mortality and morbidity and identifying populations that are most vulnerable. This project will be an applied study focused on the effects of climate change- induced extreme heat on cardiovascular morbidity and mortality in the US elderly population (age > 65 years). Our goals are to (1) conduct a national study of the cardiovascular mortality and morbidity effects of extreme heat episodes in a vulnerable population (the elderly); (2) evaluate the extent to which biological, socio-economic, and environmental factors modify vulnerability to extreme heat; and (3) estimate the impact on cardiovascular mortality and morbidity of future extreme heat episodes using temperature projections from the most up-to- date global climate model simulations for the 2020-2100 time period under a range of assumptions about pollutant emissions, population health, population age structure, climate adaptation, and climate modeling approaches. This project brings together a multi-disciplinary team with expertise in biostatistics, environmental epidemiology, atmospheric science, engineering, large database management, and climatology.